Slide 7a The Toyota Hybrid System - The Engine - 2  

• The Prius engine uses the Atkinson cycle, first proposed by James Atkinson, a British engineer, in 1887.

• Where the efficiency of an Otto cycle engine is about 35 per cent, one using the Atkinson cycle is up to 38 per cent.

• At full load, it allows a higher expansion ratio (e.g. 13:1) for a given effective compression ratio (e.g. 9.5:1).

• At part load, it requires less throttling, and hence reduces the pumping losses.

• It was first implemented by R.H. Miller, an American, who used variable opening and closing of the inlet valves.

• The Prius engine uses Toyota's VVTi system, to vary the inlet camshaft timing over 43 degrees of crank angle.

• It uses the chemically correct air/fuel ratio, to ensure the optimum performance of the three-way catalyst.

• Unlike most Otto cycle engines, the Prius engine uses no fuel enrichment (which lowers efficiency) at full throttle.

• An engine using the Atkinson cycle gives less power than with the Otto cycle, and Miller used a supercharger.

• However, in the THS, the powerful electric motor can supplement the engine at all road speeds.

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